The movement of a vehicle's attitude is determined by the expansion and contraction of the suspensions used on and rear wheels of the vehicle, and the movement is called the pitch motion.
The movement of the vehicle's attitude, associated with the pitch motion, is known to change according to the particular traveling state of the vehicle or the particular state of the road surface.
For example, accelerating or decelerating the vehicle generates a moment about its center point of gravity, called the pitch moment, in the direction that a longitudinal axis of the vehicle body changes upward or downward.
The pitch moment causes the pitch motion.
In particular, when the vehicle is braked to stop, since a braking torque is continually applied to the vehicle until it has come to a stop, the pitch moment causes a ‘nose dive’, an event that the forepart of the vehicle goes down.
In addition, when the vehicle stops, negative acceleration due to the braking torque suddenly disappears, which then leads to the vehicle releasing a burst of energy which has been stored into the suspensions during the nose dive, and results in sway-back vibration due to the nose dive.
In order to suppress these changes in attitude, caused by the pitch motion and by the nose dive due to the pitch moment, vehicles are provided with dampers to attenuate the vibration stemming from springs of the suspensions.
The vehicle is adjustable in both riding comfort and steering stability by assigning appropriate damping characteristics to the springs and dampers in the suspension system, but it is very difficult to balance between riding comfort and steering stability.
This is because, since riding comfort and steering stability are contradictory characteristics and since the actual vehicle weight changes significantly with changes in actual carrying loads such as the number of passengers and the quantity of goods loaded, optimal data that allows response to all situations is difficult to determine.
Meanwhile, there is known such a technique, as disclosed in Patent Document 1 below, that is intended for complementing a suspension's functionality by controlling a driving force of the vehicle to stabilize an attitude of the vehicle body.
The technique described in Patent Document 1 suppresses the pitch motion of the vehicle appropriately according to a particular vertical position of its wheels. More specifically, if the wheels of the vehicle body move upward, the vehicle reduces the driving torque to reduce the moment applied in the direction that the wheels move upward by road-surface repulsion, and conversely if the wheels move downward, the vehicle increases the driving torque to increase the moment applied in the direction that the wheels move upward by road-surface repulsion.